In such systems, the length of time over which the adaptive filter must operate is determined by the pulse tail length, i.e. the time taken for the pulse tail to decay to an acceptably small level. The part of the tail most distant from the main pulse occurs later in time. Conversely, the tail is created by pulses which occurred earlier in time. The adaptive filter must have a sufficient number of taps to reach the point in time where the tail has decayed by the desired amount.
If the line code used for the digital transmission has a high low-frequency content, as is the case with 2B1Q (two binary, one quaternary), the line code to be used in ISDN (Integrated Services Digital Network) Basic Access, the amplitude of the tail can still be substantial many symbol periods in time after the main pulse occurred.
In a conventional equalizer or echo canceller, a long tail requires a large number of taps, and hence faster storage access time and arithmetic processing for calculating the echo estimate and updating the coefficients. Memory and processing speed requirements increase still further if multiple samples per symbol period, or multiple coefficients per tap, are required (as for multilevel line codes). For a conventional transversal filter in which individual symbols are multiplied by respective coefficients, then summed, the number of taps required for the aforementioned long tails could result in excessive processing speeds or circuit complexity. Likewise, for a memory-type adaptive filter, the size of memory would be prohibitive, even if the memory were split, as disclosed by Kanesmasa et al in U.S. Pat. No. 4,605,826, issued Aug. 12, 1986 and entitled "Echo Canceller with Cascaded Filter Structure" and by M. Koohgoli in Canadian patent No. 1,250,035 issued Feb. 14, 1989 to Northern Telecom Limited and entitled "Split-Memory Echo Canceller", both of which documents are incorporated herein by reference.
One common method of reducing circuit complexity while handling long pulse tails is to use an Infinite Impulse Response (IIR) tap in the adaptive filter, as described in the papers by C. Mogavero, G. Norvu, G. Paschetta entitled "Mixed Recursive Echo Canceller (MREC)", in Proc. Globecom 1986, December 1986, pp 44-48, and by Takatori et al, entitled "Architecture for Fully Integrated Echo Canceller LSI based on Digital Signal Processing", in Proc. ICC 1987, June 1987, pp601-605, both of which documents are incorporated by reference. A disadvantage of this approach, however, is that it requires the decay time constant of the pulse tail to be precisely known in advance.
An object of the present invention is to provide an adaptive digital filter for an echo canceller or decision feedback equalizer in a digital data transmission system, which requires less storage and lower arithmetic processing speed than the adaptive filters discussed hereinbefore, and does not require a precise knowledge of the pulse tail characteristic.